This was Byrne's explanation for how Number One could be the ship's "most experienced officer" and still be only a Lieutenant. I think it's a good one, personally. And he included a scene where a superior officer warned her that she couldn't keep turning down promotions without suffering career consequences. Andshe eventually got kicked upstairs, anyway.
Well, technically, he refused both. It's not like they let him stay on the Enterprise and made him a Captain anyway.
Jeffrey Hunter was born 25 November 1926 and turned 38 in the year (1934) when he portrayed Pike in "The Cage". Majel Barret was born 23 February 1932 and turned 32 in the year she played Number One. Normally an officer who is five years and three months older than another would be more experienced, but possibly Number One was commissioned at a younger age and/or spent more years of her career in space, and so had more experience in space than Pike. Or possibly Pike meant Number One was the most experience officer excluding himself.
I see no problem with Pike and Number One still being lieutenants - if that is what their insignia meant - aged in their thirties considering that the alternate Picard was a junior lieutenant twenty years older than them in"Tapestry". Although maybe we should forget about "Tapestry".
The crew in "The Cage" seems very young and inexperienced, even compared to the TOS crew, but no doubt there can be plausible reasons for that, and for having only half the crew that Kirk had.